This invention relates to a frame synchronizing method and an equipment for use in a TDMA (time-division multiple-access) radio communication system which, having plural radio communication channels between plural radio stations, separates the channels from one another on a time-sharing basis and permits acquisition of a channel without using a particular reference station.
In such a TDMA system adopted as one of the multiple-access techniques for communications channels, the same frequency is commonly used, and n channels are formed by dividing the time axis as illustrated in FIG. 1 into time slots No. 0 through No. n-1 (where n is an integer) and by transmitting information with independent access to each of these n time slots constituting a frame. In order that these time slots can be used as independent communication channels, they are set so as not to overlap each other. This setting manipulation can be readily done under the condition that one of the radio stations controls all the communication channels as a reference station by inserting a frame synchronization (sync) signal on the time axis using a sync signal generator provided there.
For details of the TDMA system, reference is made to Sekimoto et al, "A Satellite Time-Division Multiple-Access Experiment", IEEE Transactions on Communication Technology, Vol. Com-16, No. 4, Aug. 1968, pp. 581-588. In this example, a specific one of plural radio stations serving as a reference station emits control signals indicating time positions on the basis of which another station finds the time position of a channel and follows a procedure to acquire the channel. In such a satellite communication system, since it is possible for every earth station to receive the control signal from the common reference station, all the stations can be given the common frame sync signal.
However, in a mobile communication system, a stationary communication system and the like, all the stations cannot necessarily receive the common signal from the reference station because of a propagation obstruction of an electromagnetic wave.
Furthermore, in case where, as shown in FIG. 2, the communication of a radio station P.sub.1 is allowed only to subscribers of the other parties C.sub.11, C.sub.12, C.sub.13 and C.sub.22 and the area Q in which channels are commonly used is greater than a communication-service available area S.sub.1, the station S.sub.1 cannot receive all the signals from all the other radio stations P.sub.2 and P.sub.3. As a result, no particular radio station can serve as a reference station with the result that the communications among the stations become impossible using the TDMA system.